Service packs have historically ALWAYS contained new features. Otherwise it's just released with Windows Update.

http://en.wikipedia.org/wiki/Windows_XP#Service_packs

> Microsoft occasionally releases service packs for its Windows operating systems to fix problems and add features.

I haven't used Windows in a few years but I remember XP SP2 included several new features. Security Center was a major one but I know there were a few others too.

I believe the wireless networking support was majorly beefed up for XP SP2.